The present invention relates generally to electronic devices which derive their power from a battery source and are sensitive in operation to the voltage potential of the battery derived power source for valid operation, and more particularly, to a voltage potential monitoring circuit for use therein to monitor the voltage potential and set a voltage potential operating threshold substantially independent of ambient temperature variation.
Contemporary battery operated electronic devices, like paging receivers, for example, generally include a portion of circuitry which may be powered directly at the voltage potential of the battery and another portion for performing digital computations, for example, which require an operating voltage potential substantially higher than that of the battery. For producing this higher voltage potential, the electronic devices generally employ a DC-DC voltage converter/regulator. The battery and higher voltage potentials are commonly referred to as B.sup.+ and B.sup.++ , respectively.
In order to conserve the power drain on the battery and increase the operating life thereof, the higher voltage potential B.sup.++ is regulated critically close to the minimum operating voltage potential of the digital computing circuits. Because of this narrow margin, it is of paramount importance to monitor the B.sup.++ voltage potential for an undervoltage condition in which the voltage potential is below a predetermined threshold and to prevent the digital computing circuits from entering into invalid states during such period of undervoltage. For this purpose, an undervoltage monitoring circuit is included in the electronic devices for monitoring the B.sup.++ voltage potential to detect an undervoltage condition. In a power on state, the undervoltage monitoring circuit is operative to suspend the operation of the digital computing circuits and maintain them at a predetermined initial operating state until the voltage potential B.sup.++ reaches an operating threshold. In a monitoring state, the undervoltage monitoring circuit detects when the B.sup.++ voltage potential drops below the operating threshold, suspends operation of the digital computing circuits, and maintains the circuits at the predetermined initial operating state until the B.sup.++ voltage potential is returned above the operating threshold.
As the designers attempt to seek longer and longer operating battery life, the margin between the regulated B.sup.++ voltage potential and the minimum operational threshold is being further reduced to the point where ambient temperature variations may have an adverse affect on both the regulation and monitoring operations, especially when the electronic device is expected to operate over a wide temperature range.
In the past, the B.sup.++ voltage regulator was permitted to have a substantial temperature coefficient over the operating temperature range as long as the undervoltage monitoring circuit was designed to operate substantially at the same temperature coefficient and not skewed therefrom. This tracking by temperature coefficients avoids the possible condition where the voltage regulator is generating one voltage potential with temperature, and the monitoring circuit is tripping at a different voltage potential because of the skewing of the temperature coefficients thereof, which condition could lead to either false undervoltage tripping by the monitoring circuit or failure to trip when needed. However, with the reduction in margin between the regulated and minimum operating voltage potentials, it becomes no longer possible to allow the voltage regulator to have a substantial temperature coefficient over the operating temperature range, but rather the regulator is being required to operate at a substantially zero temperature coefficient to ensure the generation of a voltage potential substantially independent of temperature variation. Even with temperature independent voltage regulation, the problems of temperature effects on undervoltage detection have not been completely disposed of because the undervoltage monitoring circuits have temperature coefficients which have not kept up with the temperature independent designs of their associated voltage regulators.
Accordingly, to avoid the conditions of false undervoltage tripping and failure to trip when needed over the full operating temperature range, the undervoltage monitoring circuits must operate in union with the temperature independent voltage regulating circuit. The present invention provides an undervoltage monitoring circuit for achieving this goal.